1. Field of the Invention
The present invention relates to a magnetic recording medium. More particularly, it relates to a magnetic recording medium for high density recording.
2. Description of the Prior Art
Heretofore, .gamma.-Fe.sub.2 O.sub.3, cobalt-doped or adsorbed .gamma.-Fe.sub.2 O.sub.3, magnetite and chromium dioxide have been known as magnetic powders used for magnetic recording media. However, magnetic characteristics such as a coercive force (hereinafter referring as Hc) and a maximum residual magnetic flux density (hereinafter referring as Br) of the magnetic powders will not be satisfactory for high density recording required in an industrial field. These magnetic powders are not suitable for magnetic recording of a short recording wavelength signal (1-0.5 .mu.m) or a narrow track width (about 10-20 .mu.m).
Recently, magnetic powders having characteristics suitable for high density recording have been developed. A magnetic metallic powder is one of the examples newly developed.
The typical processes for producing magnetic metallic powders are as follows;
(1) to reduce an acicular oxyhydroxide, or the acicular oxyhydroxide comprising the other metal component, or an acicular iron oxide obtained from the oxyhydroxide; or
(2) to reduce a magnetic metal salt by adding a reducing agent to an aqueous solution of the magnetic metal salt.
The magnetic metallic powders obtained by reducing it in a reducible gas flow at high temperature as the step (1) are in the form of acicular particles. The particles of the magnetic powder having a width of 300 to 600 A and an axial ratio of 5 to 15 is especially suitable for a magnetic recording medium.
The magnetic characteristics having Hc=1,3000-1,600 Oe; and saturated magnetization (hereinafter referring to as .sigma.s) of 150 to 190 emu/g. can be easily given. The magnetic metallic powders obtained by a wet process reducing process as the (2) step are in the form of the spherical beads or granule in orientation with specific spaces or in contact each other. Sometimes, the particles of the magnetic powder is in orientation under an irregular condition. The particles are chained in the form of necklace and accordingly they are called as a chain particles.
The magnetic characteristics having Hc=1,400-1,600 Oe; and a saturated magnetization .sigma.s=180-1,300 emu/g. can be easily given. The magnetic powder having each particle diameter of 150-800 A is especially suitable for a magnetic recording medium.
When the acicular particles or the chain particles are respectively used without mixing, to prepare a magnetic recording tape, the following characteristics are found.
(1) The magnetic recording tape prepared by using the acicular particles has high packed density to give Br of 3,000 to 4,000 gauss (hereinafter referring to as G).
(2) The magnetic recording tape prepared by using the chain particles has relatively low packed density to give Br of 2,000 to 3,000 gauss.
The acicular particles have superior orientation to give higher packed density whereas the chain particles have inferior orientation to give relatively lower packed density. The particles suitable for the high density recording are acicular particles as easily considered.